Process optimization and technoeconomic environmental assessment of biofuel produced by solar powered microwave pyrolysis

Microwave pyrolysis of corn stover has been optimized by Response surface methodology under different microwave power (500, 700, and 900 W) and three ratios of activated carbon additive (10, 15, and 20%) for obtaining maximum bio-oil yield followed by biochar. The optimal result has been evaluated and the environmental and techno-economic impacts of using solar-powered microwave heating have been tested. The optimal pyrolysis condition found to be 700 W microwave power and 10% of activated carbon. The yields of both bio-oil and biochar were about 74 wt% under optimal condition. The higher heat values of 26 MJ/kg and 16 MJ/kg were respectively achieved for biochar and bio-oil. The major components of bio-oil were hydrocarbons (36%) and phenols (28%) with low oxygen-containing compounds (2%) and acids (2%). Using the solar-powered system, 20,549 tonnes of CO2 can be mitigated over the lifetime of the set-up, resulting in USD 51,373 in carbon credit earnings, compared to 16,875 tonnes of CO2 mitigation and USD 42,167 in carbon credit earnings from a grid electricity system. The payback periods for solar-powered and grid-connected electrical systems are estimated to be 1.6 and 0.5 years, respectively, based on biochar and bio-oil income of USD 39,700 and USD 45,400.


Scientific Reports
| (2022) 12:12572 | https://doi.org/10.1038/s41598-022-16171-w www.nature.com/scientificreports/ Experimental set-up and procedure. The current study used a modified domestic microwave oven (LG, 28 L convection oven) with a 1-L cylindrical quartz reactor. The diagram of the experimental set-up, as well as other essential information on the experimental set-up and procedures followed, are detailed in our previous work [28][29][30] . Based on our prior work 4 , the experiments were conducted under different microwave powers of 500, 700, and 900 W and different ratios of AC additive (10,20, and 30%) with a constant reaction time of 15 min.

Optimization of pyrolysis conditions for bio-oil and biochar yields.
Response surface methodology (RSM) based on central composite design (CCD) is one of the most successful experimental designs for optimization of process parameters with the less possible number of experiments 31 . The interrelation between independent and dependent variables can be clearly known with this method. In this study, the optimized levels of microwave power (500 W, 700 W, and 900 W) and the ratios of AC as an additive (10%, 20%, 30%) were considered for investigation. The effects of these parameters were investigated on the yields of bio-oil and biochar, the desirable products for this study. After knowing the optimal condition of pyrolysis for maximum yield of bio-oil followed by biochar, the physical and thermochemical properties of both products were evaluated and characterized. The independent variables were coded as microwave power (X 1 ), and additive to biomass ratios (X 2 ). Table 2 shows the details of the coded and actual values of the experimental design. The product yields of bio-oil and biochar were chosen as dependent variables. For statistical calculations, the variables X i are coded as x i according to Eq. (1). where x i , X i , x 0 , and x are respectively the coded value for any independent variable, actual value, central value of the independent variable and step change. A 2 2 factorial CCD and 5 (4 axials and one central) points and 3 replications were employed to optimize the pyrolysis conditions for maximum yields of bio-oil and biochar. The second-degree polynomials as per the Eq. (2) were calculated using the DESIGN EXPERT version 13 software statistical package (DOE 13, Stat-Ease, Minneapolis, MN, USA), (https:// www. state ase. com/ softw are/ designexpert/) to estimate the response of the dependent variables.
where the parameters a 1 , a 2 , a 3 , a 4 , a 5 , anda 6 are regression coefficients, while Y i , X 1 , andX 2 are predicted response and the independent variables microwave power and additive to biomass ratios respectively. The experimental data from the results of the model were analyzed statistically and the fitness of the predicted model was checked by the R 2 value for providing the validation percentage of the model. Furthermore, the statistical significance of the independent parameters, considered in the study, was performed by the F-test method using a one-way analysis of variance (ANOVA) table at a significance level of 0.05 (p-value ≤ 0.05).
(1)  The Higher heat value (HHV) for raw CS and its biochar were measured by bomb calorimeter, while for bio-oil, it was calculated with the help of standard equation 32 . The water content in the bio-oil was measured using VEEGO/MATIC-D Volumetric Karl Fischer titration according to ASTM E203. The acidity, dynamic viscosity, and density of bio-oil were also measured at 25 °C. The chemical compositions of bio-oil samples were identified by gas chromatography-mass spectrometer (GC-MS) (Agilent 5890-5973). There is the provision of HP-5 capillary column in the system. The carrier gas was helium (99.999% purity) with constant flow rate of 1.2 mL/min. The volume of injected sample (1:20 split ratio) was 1 µL. The temperature of the oven was initially set at 40 °C for 40 min in the system program, then it was raised by 5 °C/min to 250 °C and held for 5 min. The injected and detector temperatures were maintained at 250 °C and 230 °C, respectively. Different compounds present in the bio-oil sample were identified by comparing their mass spectra with standard mass spectra data library of National Institute of Standards and Technology (NIST).
Techno-economic and environmental assessment. Technoeconomic analysis. The parameters considered for estimating the techno-economic assessment of the practice followed in this study include the initial cost of the experimental set-up; components used in the solar photovoltaic (PV) system, and the cost for life cycle analysis of the system. This assessment is evaluated in the scale-up system using grid electricity and solar PV power. The scale-up system is expected to process approximately 1125 tonnes of feedstock per year 33 . This requires about a 150-kW microwave reactor to process 750 kg of biomass feedstock per batch 34 with one h duration for completion 33 of the process and performing 5 batches for 10 working hours in a day 13 . In addition, the pyrolysis activities can be undertaken for 300 days per year. The practice, therefore, requires four labourers to be engaged per day to prepare the feedstock and operate the system. The parameters considered to estimate the techno-economic assessment of the MAP system are shown in Table 3.
Environmental assessment. The environmental aspect, in the form of mitigating the emissions of CO 2 and earning the carbon credit potential through the scale-up MAP system, has been assessed. For the calculation of the quantity of CO 2 emissions, it has been reported that about 1.5 kg of CO 2 is emitted into the environment by burning 1 kg of CS feedstock 1,2 . This quantity of CO 2 can be mitigated by applying the MAP process for the safe disposal of agricultural wastes like CS 28,29 . In addition, the coal-based thermal power plant generally produces about 0.98 kg of CO 2 /kWh. Taking distribution and transmission losses into account, a thermal power plant based on coal materials is expected to emit about 1.57 kg of CO 2 /kWh 35 . This quantity of CO 2 emissions can also be mitigated by integrating solar PV electricity, clean energy, with the microwave pyrolysis practice.

Results and discussion
Response surface analysis. Figure 2 shows the influence of quadratic terms of the dependent parameters on bio-oil and biochar yields along with predicted versus actual responses for each product when using AC as an additive. The quadratic model equations for response Y Bio-oil and response Y biochar were obtained using the results of the experiments and presented in Eqs. (3) and (4), respectively. The relation between predicted versus actual responses for bio-oil and biochar when using AC as an additive were presented in Fig. 2a (right) and 2b (right) respectively.   Table A-1, the ANOVA results, the P-values for bio-oil (< 0.0001) and for biochar (0.0004) were less than 0.05, suggesting that these equations for the resulted models were statistically significant at a 95.0% confidence interval (p < 0.05) to describe the yields of bio-oil and biochar. The coefficients of determination (R 2 ) for the bio-oil and biochar response (Eqs. 3 and 4) were found to be 0.97 and 0.95 respectively. This implies that microwave power and different AC additive to biomass ratios under the studied ranges had a significant catalytic influence on bio-oil and biochar distribution yields during MAP of CS.
Products yield and their characteristics. The products, yield and their characteristics have been evaluated under optimal pyrolysis condition (700 W and 10% AC). The optimal condition has been established from the optimization process with the aim of maximum bio-oil yield followed by biochar (the desirable products of the study). Table 4 shows the main properties of biochar and bio-oil from microwave pyrolysis of CS under optimal pyrolysis condition. Under optimal condition, the yields of the products were found to be 45 wt%, 28 wt%, and 27 wt% for bio-oil, biochar, and gas, respectively (Table 4). This means, under optimal pyrolysis condition, the bio-oil and biochar yields represent about 74 wt% of the total weight of raw CS used in the experiments. This clearly indicates that the major percentage of raw CS feedstock is converted into solid and liquid products as compared to the gas product (26 wt%). The comparison between the product yields under different pyrolysis condition can be found in the supplementary materials in Appendix A Table A-2. The biochar with HHV of 26 MJ/kg was achieved compared to 16.7 MJ/kg for raw CS and can be used as a solid biofuel in many applications.
In addition, the energy yield from biochar with respect to the energy content in the raw CS was about 44%. Also, the biochar produced from the present study contains low content of nitrogen and was free of sulphur compared with raw CS biomass. This indicates that the biochar produced can be utilised as an environmentally friendly and clean solid biofuel to replace conventional coal, reducing greenhouse gas emissions such as CO and CO 2 , as well as harmful gas emissions such as SOx and NOx into the atmosphere.
For bio-oil, Table 4 shows that the HHV and energy yield from bio-oil reached 16.4 MJ/kg and 44% respectively which is due to the higher yield of bio-oil. Also, the bio-oil free from sulphur and nitrogen contents represents a beneficial feature for its use as a cleaner biofuel with respect to reduced emissions of greenhouse gases to the atmosphere. The moderate values of pH (5.3), viscosity (2.89 mPa s), and water content of 38.92% of the bio-oil were also achieved. The GC-MS analysis of the bio-oil revealed that the chemical composition was classified into 9 groups as well as the unidentified compounds (as per a match factor of 85%). These groups are acids, phenols, furans, guaiacols, esters, alcohols, hydrocarbons, ketones/aldehydes, and nitrogenous compounds. The contents of hydrocarbons and phenols in the bio-oil were up to 36% and 28% respectively with low oxygencontaining compounds (2%), low acids (2%), esters (1.2%), and alcohols (2%), in addition to furans content of 6% and ketones/aldehydes of 7%. The details of the bio-oil chemical composition analysed by GC-MS can be found in the Supplementary Materials Appendix A Table A-3. As a result of the increased quality of both biooil and biochar, produced from this study, both the products can be considered as a source of biofuel in many applications.  33 of the process and performing 5 batches for 10 working hours in a day 13 . The pyrolysis activities can be undertaken for 300 days per year. The electricity required to operate the system was calculated to be 780 kWh per day [(150 kW microwave reactor + 4 kW water pump + 2 kW temperature monitoring set-up) × reaction time per batch (1 h) × number of batches per day (5)]. In the case of operating the system by solar PV system, the required PV system should be sized accordingly. The sizing and cost estimation of solar PV system for both the systems have been presented in the Table 5 as per the calculations and explained in supplementary materials Appendix B. Table 6 illustrates the estimation of total expenditures with respect to capital and operating costs as well as the revenues to be earned from both the production systems. The capital cost has been estimated based on the studies carried out by the previous researcher in microwave pyrolysis 33 considering a 3% escalation factor per year for the price of the components. Operating costs, such as feedstock and pyrolytic product transportation, maintenance, and other miscellaneous costs, are estimated as percentages of the capital cost (see Table 3). The cost of the electricity required to operate the system (in case of grid electricity used) has also been calculated.
The cost of the electricity, when using grid electricity, was estimated to be USD18720 per year (780 kWh per day × USD0.08/kWh (Rs6/kWh) × 300 working days/year). Feedstock cost is estimated to be USD40 per tonne 1 . The proposed scale-up system is assumed to process about 1125 tonnes (750 kg per batch × 5 batches/day × 300 working days/year) as seen in Table 3. Percentages in the yield of pyrolytic products were estimated to be 45 wt%, 28 wt%, and 27 wt% for bio-oil, biochar, and pyrolytic gas, respectively, as per our experimental condition when pyrolyzed at 700 W microwave power and using AC additive as presented in Table 4 and Table A-2 in the "Supplementary materials S1".
These pyrolysis conditions were selected as it produced the higher yields of bio-oil along with biochar (desirable products for the present study) with high quality. Also, the AC additive can be used as an additive repeatedly used in the MAP process system for 25 days 4,24 . The cost of AC is USD 2.4/kg and the required quantity per batch is 75 kg (750 kg feedstock/batch × 10/100 ratio of AC).
Thus, the total cost of using AC was estimated to be USD180 per 25 days (75 kg AC × USD2.4/kg). The total cost of AC per year is estimated to be USD2160 per year (USD180 per 25 days × 300 working days per year). The revenues to be generated by trading the pyrolytic products i.e., bio-oil and biochar have been estimated, based on the current prevailing prices as per the Egyptian market, considering their quality achieved in this study. The bio-oil produced in this study has a high proportion of hydrocarbons (36%). Phenols compounds accounted for the second-highest proportion (28%). In addition, the bio-oil had a low concentration of guaiacols compounds (2%) and acids (2%). Thus, it is considered to be a good source for phenols and chemical extraction. Therefore, the cost of the bio-oil is estimated to be USD1 per kg and the density of the bio-oil is estimated to be 1.036 g/mL (~ 1 g/mL), thus price is USD1 per liter 33,36,37 . The annual bio-oil production is estimated to be 506,250 kg (liter) (1125 tonne feedstock per year × 1000 × 45 wt%/100 bio-oil yield).
For biochar, the selling price is estimated to be USD500/tonne 33,34,38 . The annual biochar production was estimated to be 315 tonne biochar per year (1125 tonne/year × 28 wt%/100 = 315 tone biochar/year) as per the optimal pyrolysis conditions. Environmental assessment. The emission of CO 2 can be mitigated by applying the MAP practice operated by solar PV electricity for more sustainable of reutilize the agriculture residues. As the result from the practice, the mitigation of CO 2 emission during 10-year lifetime of the set-up was estimated to be 20,549 tone/life for solarpowered system out of the calculation from the following equation: Mitigation of CO 2 emission = 1.57 kg CO 2 from coal based thermal power plant/kWh/1000 × E out kWh/ year × life of set-up) + (1.5 kg CO 2 emitted per kg of agricultural residue burning × kg of feedstock pyrolyzed per year × life of set˗ up).
Whilst, in the case of grid electricity system, the mitigation of CO 2 emission was estimated to be 16,875 tonne/ life for grid electricity system out of the calculation from the following equation: Mitigation of CO 2 emission = 1.5 kg CO 2 emitted per kg of agricultural residue burning × kg of feedstock pyrolyzed per year × life of set˗ up. www.nature.com/scientificreports/ For the estimation of credit earning by mitigation of CO 2 , it is reported that about USD 2.5 per mitigating one tonne of CO 2 is credit earned (www. ecx. eu). Therefore, the carbon credit earned from the mitigation of CO 2 to the environment is estimated to be USD51373 per life and USD42187 per life for solar-powered and grid electricity respectively out of the calculation from the following equation: Carbon credit earned = USD2.5 per tonne × CO 2 emission mitigated by the set-up per life. As the result from the techno-economic and environmental analyses, the scaling-up of the MAP system for treating the agro-residues can be economically and environmentally beneficial. It is assessed that by using grid electricity, the payback period would be very nominal (0.5 year) compared to using a solar-powered system (1.9 years), resulting into earning more profit from the set-up during the rest of the life period of the system. Similarly, the benefit-cost ratio would be higher, causing the production system to be more cost-effective. This is because of the higher capital cost in the case of the solar-powered system. However, the mitigation of CO 2 emission during the lifetime of the set-up is assessed to be more 20,549 tonnes/life when using the solar-powered system compared to 16,875 tonnes/life by the grid electricity system. This results into more credit earned from carbon dioxide mitigation i.e. USD51373 per life for solar PV powered system and USD42187 per life for grid electricity powered system. The MAP system could be a serviceable practice for mitigation of emission of CO 2 , causing the effective utilization of agro-residues and increased sustenance for the environment. Table 6. Techno-economic and environmental assessment for scaling-up microwave pyrolysis production system of corn stover. [ a Based on Table 5, b Total cost of the microwave pyrolysis system include the reactor, microwave system, condensation system, N 2 gas, and temperature monitoring system was estimated based on previous work in microwave pyrolysis 33 , considering 3% escalation factor per year (https:// fred. stlou isfed. org/), [200000 + (200,000 × 3/100 × 12 year)] = USD272000), c Annual yield of the biochar was 28 wt% from annual feedstock consumed (1125 tonne/year × 28 wt%/100 = 315 tone biochar/year), d Annual yield of the bio-oil was 45 wt% from annual feedstock consumed (1,125,000 × 45 wt%/100 = 506,250 kg bio-oil/year). Considering the density of the bio-oil is 1.036 g/mL (~ 1 g/mL) Table 4, thus the annual bio-oil yield was estimated to be 506,250 L/year, e Selling price of biochar as fertilizer or soil conditioner and fuel in domestic cooking was estimated to be USD500/tonne, f Selling price of bio-oil for chemical and phenol extraction was estimated to be USD1/L, g Net annual income is equal to total annual revenues-total operation cost].

Conclusion
In the present study, the environmental and techno-economic aspects of biochar and bio-oil produced by the pyrolysis of corn stover using solar-powered microwave heating and activated carbon as an additive were investigated. The process is first optimized for producing maximum bio-oil yield, followed by biochar. The yields of both bio-oil and biochar were found to be maximal under optimal condition of 700 W microwave power and 10% of AC additive. The result showed that the cost-effective and environmentally sustainable integration of the microwave-assisted pyrolysis process with solar photovoltaic power to produce biofuel and value-added products is viable. In comparison to 16,875 tonnes of CO 2 mitigation and USD 42,167 in carbon credit earnings from a grid electricity system, the solar-powered system may mitigate 20,549 tonnes of CO 2 during the lifetime of the set-up, resulting in USD 51,373 in carbon credit earnings. This will assist to reduce the power required for the processes and eliminate the use of fossil-fuel-based electricity, resulting in additional advantages for users and a more environmentally friendly approach to the technology's long-term sustainability. The findings from the present study would ultimately provide appropriate information regarding the technical feasibility and economic viability of the practice for its acceptability among the users. Future research should also pay attention to more investigations on a large scale, making the process suitable for adoption among the producers and making it more beneficial.

Data availability
All data generated or analyzed during this study are included in this published article.